Disable a Field but Save to Database

Hi, I have a field: deposit_total and I am using JavaScript to calculate the field. I don't want users to input in the field so I have to disable it so they can see the total.
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When I do this it will not save to the Database. What is the best way to save this to the database?

My code:

$("#rent_advance,#room_bond,#dps_deposit").on("change", function(){
 $("#deposit_total").val(parseInt($("#rent_advance").val()) + parseInt($("#room_bond").val()) + parseInt($("#dps_deposit").val()));
});

Calculate it again using PHP after submit.,

I found a simple way to do it, I added an event on the Submit button to re-enable it. Works!
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No that's terrible. If you do it that way I can just manually set the value and send it

Hi spytech,
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Disabled fields are not submitted - you need to set it to readonly instead.
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And Healyhatman is correct - re-check the calculation after the form is submitted. It's easy enough to edit readonly inputs in the browser if you know what you are doing.
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Bob

Hi, this works becuase it is staff who are filling this in so they will have no reason to fiddle with the amounts.
Thanks.